Now that support for Windows XP has ended, it has started to show a notice when it loads.

This can be disable by ticking the box shown. For most home users, the end of support for XP is not a big issue, see my earlier post on End of support for Windows XP.
Microsoft are using the word “support” to mean a very specific thing: “this product will no longer be developed, no new features will be added and no existing flaws will be fixed”. So what you have now is what you’ll always have. It will continue to work and you’re entitled to use it.
As far as Windows XP goes, that’s fair enough.
Microsoft have also ended support for their anti-virus product Microsoft Security Essentials (MSE) on XP. Again this means the product won’t be further developed, but it will continue to work. In addition, Microsoft has said they they will continue to provide “signature updates” for MSE until 14th July 2015, so it will continue to detect new viruses and other malware until then. This is fine for most people, and when 14th July 2015 rolls around, there may well be good replacements available for MSE from other vendors.
Of course, if the virus was exploiting an underlying flaw in XP or MSE, Microsoft won’t fix that flaw. But most of the malware I see doesn’t exploit underlying flaws, so it’s not a major problem. It’s certainly not the biggest security risk users face.
So given all this, Microsoft’s warnings are technically true, but not nearly as calamitous as the sound. Additionally, MSE is now permanently red, says “your PC is at risk”, and shows a pop-up in the notification area when you start Windows:

The main page of MSE repeats the warning:

Although technically true (aren’t all PCs at risk all the time anyway … your house could catch fire at any time and destroy your PC) this doesn’t mean what most normal people would think it means, and they should not worry. Note it also says that the virus and spyware definitions are up-to-date, which is fine. If you look at the Update tab you can see the date of the virus detection files (in this case created by Microsoft on 9 April at 22:03and collected by this copy of MSE on 10 April at 04:56):

So Microsoft is indeed continuing to provide virus detection updates to their “unsupported” product.
Of course, Windows XP is an old operating system, usually running on an old PC, and users will want to move to something faster and more modern in due course. The best way to move forward for most people will be to buy a new PC, which will come with either Windows 7 or Windows 8 (both are still widely available, Windows 7 has not been discontinued). For most of my users, Windows 7 is a far better choice than Windows 8 (which is very different and has not been well received). There is no doubt that Windows 7 is a better, more secure operating system that Windows XP, so users should certainly consider upgrading, but there’s no need to rush into it just because support for XP has ended.